Magenta toner and production process thereof

ABSTRACT

The invention relates to a magenta toner having magenta-colored resin particles comprising at least a binder resin and a magenta colorant. The magenta pigment is a magenta pigment composed of a combination of C.I. Pigment Red 31 and C.I. Pigment Red 150.

TECHNICAL FIELD

The present invention relates to a magenta toner for developing anelectrostatic latent image formed on a photosensitive member by anelectrophotographic process or electrostatic recording process, and moreparticularly to a magenta toner which can be used in formation offull-color images, and a production process thereof.

In the present invention, magenta-colored resin particles comprising atleast a binder resin and a magenta colorant may be referred to as“magenta toner particles” in some cases. Magenta-colored resin particlesobtained by a polymerization process may also be referred to as“magenta-colored polymer particles” in some cases. In the presentinvention, developers containing the magenta-colored resin particlessuch as a one-component developer comprising the magenta-colored resinparticles and an external additive (for example, a flowability improver)and a two-component developer comprising the magenta-colored resinparticles and carrier particles are called “magenta toners”.

BACKGROUND ART

In an image forming apparatus such as an electrophotographic apparatusor electrostatic recording apparatus, an electrostatic latent imageformed on a photosensitive member is first developed with a developer(toner). After the toner image formed on the photosensitive member isthen transferred to a transfer medium such as paper as needed, the imageis fixed thereto by any of various methods such as heating, pressing anduse of solvent vapor.

As such image forming apparatus, a digital full-color copying machineand a digital full-color printer are put to practical use. In thedigital full-color copying machine, after a colored image original issubjected to color separation through respective filters of blue, greenand red, electrostatic latent images of the respective colors composedof a dot diameter of 20 to 70 μm corresponding to the colored imageoriginal are formed. These electrostatic latent images are developedwith respective toners of yellow, magenta, cyan and black colors to forma full-color image making good use of a subtractive mixture effect.

In recent years, a demand for formation of full-color images with highimage quality and high definition has more and more increased. In orderto improve color reproducibility in particular, it is desirable that animage using color toners can be printed with a hue equivalent toprinting with inks. As colorants for magenta toner, have heretofore beengenerally used organic pigments such as quinacridone pigments, perillenepigments, thioindigo pigments and β-oxynaphthenic anilide pigments(i.e., Naphthol AS pigments). Among these, the quinacridone pigments arewidely used in that they are excellent in light resistance, heatresistance and transparency. With respect to the quinacridone pigments,it has been proposed to use a quinacridone pigment and anotherquinacridone pigment in combination or use a quinacridone pigment andanother magenta pigment in combination for the purpose of improvingtoner properties.

For example, Japanese Patent Application Laid-Open No. 10-312088 hasproposed a magenta toner making combined use of C.I. Pigment Red 122 andC.I. Pigment Red 57:1 as a colorant. Japanese Patent ApplicationLaid-Open No. 2000-181144 has proposed a magenta toner making combineduse of a dimethylquinacridone pigment (i.e., C.I. Pigment Red 122) andat least one red pigment having negatively charging ability or weaklypositively charging ability as a colorant. Japanese Patent ApplicationLaid-Open No. 2002-91086 has proposed a magenta toner making combineduse of a quinacridone pigment and a Naphthol AS pigment or β-naphthollake pigment as a colorant.

However, these magenta toners has involved a problem that magenta tonerparticles are easy to be broken by contact between the magenta tonerparticles or stress between a feed roll and a development roll, orbetween a development and a photosensitive member in an image formingapparatus. When the magenta toner particles are broken, the flowabilityand printing density of the magenta toner are lowered.

In addition, C.I. Pigment Red 57:1 that is a magenta pigment used inrespective Examples in Japanese Patent Application Laid-Open No.10-312088 and Japanese Patent Application Laid-Open No. 2002-91086 ispoor in light resistance, and so the resulting image may be faded withtime in some cases.

Further, since C.I. Pigment Red 5 and C.I. Pigment Red 209 that aremagenta pigments used in respective Examples in Japanese PatentApplication Laid-Open Nos. 2000-181144 and 2002-91086 are chlorineatom-containing compounds, there is a possibility that dioxin may begenerated when paper, on which images have been formed, is incinerated.

Japanese Patent Application Laid-Open No. 2000-156795 discloses magentatoners containing C.I. Pigment Red 122, C.I. Pigment Violet 19 and C.I.Pigment Red 150 as colorants. However, these magenta toners are low inprinting density and may cause fog in some cases.

As described above, when a quinacridone pigment is used as a magentacolorant, light resistance is improved compared with the case whereanother pigment is used. However, such a pigment tends to lower theprinting density of the resulting magenta toner. In order to raise theprinting density using the quinacridone pigment, it is necessary to usethe pigment in a great amount. When the quinacridone pigment is used ina great amount, however, not only the cost of the resulting magentatoner is increased, but also the fixing ability of the toner is liableto lower.

On the other hand, it is investigated to use a Naphthol AS pigment alonewithout using any quinacridone pigment to provide a magenta toner.Naphthol AS pigments are classified into a mono-azo system and a dis-azosystem. However, mono-azo pigments are generally widely adopted.

Japanese Patent Application Laid-Open Nos. 2000-81734 and 2002-182433disclose magenta toner particles comprising C.I. Pigment Red 31 as amono-azo pigment in Examples thereof. These magenta toner particles areapplied to formation of images by a two-component development system asa two-component developer combined with carrier particles. However, theinvestigation by the present inventors has revealed that when thesemagenta toner particles are applied to formation of images by anon-magnetic one-component development system, not only the density ofan image printed becomes low, but also hot offset occurs, and moreoverstorage stability becomes poor.

Japanese Patent Application Laid-Open No. 2002-72569 discloses a magentatoner comprising C.I. Pigment Red 150 as a mono-azo pigment. However,the investigation by the present inventors has revealed that not onlythis magenta toner can form only images having a hue far from a hue byprinting with inks and is poor in color reproducibility, but also it islow in printing density and poor in low-temperature fixing ability andcauses fog under high-temperature and high-humidity conditions.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a magenta tonerwhich is high in printing density, causes no fog and can reproduce a hueequivalent to printing with inks.

Another object of the present invention is to provide a magenta tonerhas low-temperature fixing ability and causes no fog even under severeenvironments of low temperature and low humidity, and high temperatureand high humidity.

A further object of the present invention is to provide a magenta tonerwhich prevents magenta-colored resin particles from breaking in an imageforming apparatus to low flowability, prevents an image obtained byprinting from fading and has little possibility of causing anenvironmental problem even when a transfer medium, on which an image hasbeen formed with the toner, is incinerated.

A still further object of the present invention is to provide aproduction process of a magenta toner having such excellent propertiesas described above.

The present inventors have carried out an extensive investigation with aview toward achieving the above-described objects. As a result, it hasbeen found that the above-described objects can be achieved by usingspecific magenta pigments in combination as a magenta colorant in amagenta toner having magenta-colored resin particles comprising at leasta binder resin and the magenta colorant. The present invention has beenled to completion on the basis of this finding.

According to the present invention, there is thus provided a magentatoner having magenta-colored resin particles comprising at least abinder resin and a magenta colorant, wherein the magenta colorant is amagenta pigment composed of a combination of C.I. Pigment Red 31 andC.I. Pigment Red 150.

According to the present invention, there is also provided a process forproducing a magenta toner having magenta-colored resin particles, whichcomprises Step 1 of preparing a polymerizable monomer compositioncontaining at least a polymerizable monomer and a magenta colorant andStep 2 of polymerizing the polymerizable monomer composition in anaqueous dispersion medium to form the magenta-colored resin particles,wherein a magenta pigment composed of a combination of C.I. Pigment Red31 and C.I. Pigment Red 150 is used as the magenta colorant.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Magenta-Colored Resin Particles

The magenta-colored resin particles (magenta toner particles) arecolored resin particles comprising a binder resin and a magenta pigmentas essential components. As examples of the binder resin, may bementioned various resins heretofore used as binder resins in atechnological field of toners, such as polystyrene, styrene-n-butylacrylate copolymers, polyester resins and epoxy resins.

(1) Magenta Colorant:

The magenta colorant used in the present invention is a magenta pigmentcomposed of a combination of C.I. Pigment Red 31 and C.I. Pigment Red150. The combination of these two pigments also includes the form of amixed crystal in addition to the form of a mixture. The use of themagenta pigment composed of the combination of these two pigments canprovide a magenta toner capable of forming images having a hueequivalent to magenta in printing with inks. Since the magenta pigmentcontains no chlorine atom, there is little possibility that anenvironmental problem by generating dioxin may be caused even when atransfer medium such as paper, on which an image has been formed withthe resulting magenta toner, is incinerated.

The surface of the magenta pigment used in the present invention may betreated with a rosin compound or the like for the purpose of improvingthe dispersibility of the pigment in the resulting magenta-colored resinparticles. As a method for the surface treatment, may be adopted anypublicly known method such as mixing of the magenta pigment with atreating agent. However, if the amount of the treating agent to themagenta pigment is too great, the resulting magenta toner may cause fogunder high-temperature and high-humidity conditions in some cases.Accordingly, the amount of the treating agent used is preferably at most20 parts by mass, more preferably at most 10 parts by mass, particularlypreferably at most 5 parts by mass per 100 parts by mass of the magentapigment.

A proportion of C.I. Pigment Red 31 to C.I. Pigment Red 150 used iswithin the range of preferably from 30:70 to 80:20, more preferably from40:60 to 70:30, particularly preferably from 50:50 to 60:40 in terms ofa mass ratio.

The magenta pigment composed of these two pigments is generally usedwithin the range of 1 to 10 parts by mass in total per 100 parts by massof the binder resin.

(2) Charge Control Agent:

In the present invention, a charge control agent is preferably containedin the magenta-colored resin particles for the purpose of controllingthe charge level of the resulting magenta toner. Among charge controlagents, charge control resins are particularly preferred because theyare high in compatibility with the binder resin and colorless and canprovide a magenta toner stable in charging ability in the case ofcontinuous high-speed color-printing.

As the charge control resins, are preferred quaternary ammonium (salt)group-containing copolymers prepared in accordance with a processdescribed in, for example, Japanese Patent Application Laid-Open Nos.63-60458, 3-175456, 3-243954 and 11-15192 and sulfonic (salt)group-containing copolymers prepared in accordance with a processdescribed in, for example, Japanese Patent Application Laid-Open Nos.1-217464 and 3-15858.

A monomer unit having a quaternary ammonium (salt) group or sulfonic(salt) group contained in these copolymers is contained in a proportionof 0.5 to 15% by mass, preferably 1 to 10% by mass in each copolymer.When the content of these monomer units falls within the above-describedrange, the charge level of the resulting magenta toner is easy to becontrolled, and the occurrence of fag can be lessened.

The weight average molecular weight of the charge control resin iswithin the range of generally 2,000 to 50,000, preferably 4,000 to40,000, more preferably 6,000 to 30,000. When the weight averagemolecular weight of the charge control resin used falls within thisrange, the saturation and transparency of the resulting magenta tonercan be highly retained.

The glass transition temperature of the charge control resin is withinthe range of generally 40 to 80° C., preferably 45 to 75° C., morepreferably 45 to 70° C. When the glass transition temperature of thecharge control resin used falls within this range, a balance betweenstorage stability and fixing ability in the resulting magenta toner canbe improved.

The proportion of the charge control resin used is within the range ofgenerally 0.01 to 20 parts by mass, preferably 0.1 to 10 parts by massper 100 parts by mass of the binder resin.

(3) Parting Agent:

In the present invention, a parting agent is preferably contained in themagenta-colored resin particles. Examples of the parting agent includepolyolefin waxes such as low molecular weight polyethylene, lowmolecular weight polypropylene and low molecular weight polybutylene;natural plant waxes such as candelilla, carnauba, rice, Japan wax andjojoba; petroleum waxes such as paraffin, microcrystalline andpetrolatum, and modified waxes thereof; synthetic waxes such asFischer-Tropsch wax; and polyfunctional ester compounds such aspentaerythritol tetramyristate, pentaerythritol tetrapalmitate anddipentaerythritol hexamyristate. These parting agents may be used eithersingly or in any combination thereof.

Among these parting agents, synthetic waxes, terminal-modifiedpolyolefin waxes, petroleum waxes and polyfunctional ester compounds arepreferred. Among the polyfunctional ester compounds, pentaerythritolesters whose endothermic peak temperatures upon heating fall within therange of 30 to 200° C., preferably 40 to 160° C., more preferably 50 to120° C. in a DSC curve as determined by a differential scanningcalorimeter (DSC), and dipentaerythritol esters whose endothermic peaktemperatures fall within the range of 50 to 80° C. are preferred fromthe viewpoint of improving a balance between the fixing ability and theparting property in the resulting magenta toner. Among thepolyfunctional ester compounds such as pentaerythritol esters whoseendothermic peak temperatures upon heating fall within the range of 30to 200° C., and dipentaerythritol esters whose endothermic peaktemperatures fall within the range of 50 to 80° C., those having amolecular weight of at least 1,000, a solubility of 5 parts by mass in100 parts by mass of styrene at 25° C. and an acid value of at most 10mgKOH/g are particularly preferred because they exhibits a marked effecton lowering of the fixing temperature of the resulting magenta toner.The endothermic peak temperature is a value measured in accordance withASTM D 3418-82. Since the low-temperature fixing ability of theresulting magenta toner is improved by using these parting agents, and afixing region becomes broad (namely, offset margin becomes great), thismagenta toner can be applied to not only an image forming apparatusheretofore used, on a fixing roll of which an anti-offset liquid hasbeen coated, but also an image forming apparatus that is not coated withsuch an anti-offset liquid.

The proportion of the parting agent used is within the range ofgenerally 0.5 to 50 parts by mass, preferably 1 to 20 parts by mass per100 parts by mass of the binder resin.

(4) Magenta-Colored Resin Particles with Core-Shell Structure:

The magenta-colored resin particles can be provided as magenta-coloredresin particles having a core-shell structure obtained by respectivelyusing two different resin components for interiors (core particles) andexteriors (shell layers) of particles in combination. A magenta tonercomprising such magenta-colored resin particles having the core-shellstructure may be called a capsule type magenta toner in some cases. Inthe particles of the core-shell structure, the interiors (coreparticles) are formed by a resin component having a low softening pointor low glass transition temperature, and then coated with a resincomponent having a softening point or glass transition temperaturehigher than the first-mentioned resin component, whereby a balancebetween lowering of fixing temperature and prevention of aggregation(prevention of blocking) during storage can be improved.

In the magenta-colored resin particles of the core-shell structure, thecore particles are generally formed by a binder resin, a magentapigment, a charge control agent, a parting agent and the like, and theshell layer are formed by a resin (polymer) alone.

In the magenta-colored resin particles of the core-shell structure, thevolume average particle diameter (dv) of the core particles is withinthe range of preferably 3.0 to 12.0 μm, more preferably 4.0 to 10.0 μm,particularly preferably 5.0 to 8.0 μm. The particle diameterdistribution dv/dp represented by a ratio of the volume average particlediameter dv to the number average particle diameter dp of the coreparticles is within the range of preferably 1.0 to 1.3, more preferably1.0 to 1.2.

No particular limitation is imposed on a mass ratio of the coreparticles to the shell layers in the magenta-colored resin particles ofthe core-shell structure. However, it is generally within the range of80/20 to 99.9/0.1. When the proportion of the shell layers is controlledwithin the above-described range, the resulting magenta toner can havegood storage stability and low-temperature fixing ability incombination.

The average thickness of the shell layers in the magenta-colored resinparticles of the core-shell structure is within the range of preferably0.001 to 1.0 μm, more preferably 0.003 to 0.5 μm, particularlypreferably 0.005 to 0.2 μm. If the thickness of the shell layers is toogreat, the fixing ability of the magenta toner may possibly bedeteriorated. If the thickness is too small to the contrary, the storagestability of the toner may possibly be deteriorated. In themagenta-colored resin particles of the core-shell structure, all thesurfaces of the core particles are not necessarily closely coated with ashell layer, but a part of the surfaces of the core particles may becoated with the shell layer so far as aggregation upon storage can beprevented.

The average particle diameter of the core particles and the averagethickness of the shell layers in the magenta-colored resin particles ofthe core-shell structure can be determined by directly measuring thesize and shell thickness of each of plural particles selected at randomfrom an electron photomicrograph thereof to calculate out respectiveaverage values when they can be observed through an electron microscope.If the core particle and the shell layer are difficult to clearlydistinguish by observation through the electron microscope, an averageparticle diameter of core particles obtained upon the production of thetoner is measured. The average thickness of the shell layers can becalculated out on the basis of the measured value of the averageparticle diameter of the core particles and the amount of theshell-forming material used in the formation of the shell layers, suchas a polymerizable monomer.

(5) Properties of Magenta-Colored Resin Particles:

The volume average particle diameter dv of the magenta-colored resinparticles is within the range of preferably 3.0 to 12.0 μm, morepreferably 4.0 to 10.0 μm, particularly preferably 5.0 to 8.0 μm. Thevolume average particle diameter of the magenta-colored resin particlesof the core-shell structure is also desirably within the same range asdescribed above. If the volume average particle diameter of themagenta-colored resin particles is too small, the flowability of theresulting magenta toner is lowered, and so its transferability may belowered, blur may occur, or the printing density may be lowered. If thisvolume average particle diameter is too great, fog and flying-off of theresulting toner occur, and the resolution of an image formed with such atoner is deteriorated.

No particular limitation is imposed on a particle diameter distributiondv/dp that is a ratio of the volume average particle diameter dv to thenumber average particle diameter dp in the magenta-colored resinparticles. However, it is preferably within the range of 1.0 to 1.3,more preferably 1.0 to 1.2. If the particle diameter distribution is toogreat, the resulting toner tends to cause blur or lower transferability,printing density and resolution. The volume average particle diameterand particle diameter distribution of the magenta-colored resinparticles can be measured by means of, for example, a Multisizer(manufactured by Beckmann Coulter Co.).

In the magenta-colored resin particles, an average spheroidicity rl/rsobtained by dividing a length rl of each particle by its breadth rs iswithin the range of preferably 1.0 to 1.3, more preferably 1.0 to 1.2,particularly preferably 1.0 to 1.15. If the average spheroidicity is toogreat, the transferability of the toner may be lowered in some cases.The average spheroidicity of the magenta-colored resin particles can beeasily controlled within the above range by using a phase-inversionemulsification process, dissolution and suspension process,polymerization process or the like.

2. Magenta Toner

The magenta toner according to the present invention may be formed bythe magenta-colored resin particles (magenta toner particles) alone. Inorder to improve cleaning ability, charging ability, flowability andstorage stability of the resulting toner, however, the toner isgenerally prepared by adding an external additive to the magenta-coloredresin particles. The magenta toner comprising the magenta-colored resinparticles and the external additive is suitable for use as anon-magnetic one-component developer. A two-component developer can beprovided by combining the magenta-colored resin particles with carrierparticles. An external additive may also be applied to themagenta-colored resin particles used in the two-component developer.

The external additive may be applied to the surfaces of themagenta-colored resin particles by stirring these components in a mixersuch as a Henschel mixer, or a part thereof may be embedded therein.

Hexahedral inorganic fine particles are preferably used as the externaladditive. The hexahedral inorganic fine particles have a form of ahexahedron such as a cube or rectangular parallelopiped. They may besomewhat deformed by, for example, rounding apexes thereof. A lengthratio of the longest arris to the shortest arris among arrises making upthe hexahedron is preferably within the range of 1 to 2, or is morepreferably 1.

No particular limitation is imposed on the chemical structure of thehexahedral inorganic fine particles. However, calcium carbonate ismentioned as a representative thereof. No particular limitation isimposed on the volume average particle diameter of the hexahedralinorganic fine particles. However, it is within the range of preferably0.05 to 10 μm, more preferably 0.1 to 5 μm. If this volume averageparticle diameter is too small, the cleaning ability of the resultingtoner is lowered. If the volume average particle diameter is too greatto the contrary, the flowability of the toner is lowered, and so blurmay occur, or image defects may be caused in some cases.

The hexahedral inorganic fine particles are preferably subjected to ahydrophobicity-imparting treatment. As the hexahedral inorganic fineparticles subjected to the hydrophobicity-imparting treatment, may beused a commercially available product. However, they may be prepared bysubjecting untreated hexahedral inorganic fine particles to ahydrophobicity-imparting treatment with a silane coupling agent,silicone oil, fatty acid, fatty acid metal soap or the like.

No particular limitation is imposed on the amount of the hexahedralinorganic fine particles used. However, it is within the range ofpreferably 0.05 to 5 parts by mass, more preferably 0.1 to 3 parts bymass per 100 parts by mass of the magenta-colored resin particles. Ifthe amount used is too little, the effect of improving the cleaningability becomes small. If the amount is too great to the contrary, theflowability of the resulting toner is lowered, and so blur may occur insome cases.

Besides the hexahedral inorganic fine particles, fine particles, whoseshape is spherical or indefinable, are preferably used as an externaladditive. As the fine particles whose shape is spherical or indefinable,any of inorganic fine particles and organic fine particles may be used.However, inorganic fine particles are preferred from the viewpoint ofcontrolling the flowability and charging ability of the resulting toner.These fine particles may be used either singly or in any combinationthereof.

Examples of the inorganic fine particles, whose shape is spherical orindefinable, include silica, titanium oxide, aluminum oxide, zinc oxide,tin oxide, barium titanate and strontium titanate. Among these, silicais preferred because fog upon printing can be lessened.

No particular limitation is imposed on the volume average particlediameter of the inorganic fine particles. However, it is within therange of generally 5 to 500 nm, preferably 5 to 100 nm, more preferably7 to 50 nm. If this volume average particle diameter is too small, theresulting toner undergoes charge-up at a low temperature and a lowhumidity to lower its printing density. If the volume average particlediameter is too great to the contrary, the flowability of the resultingtoner is lowered, and so blur may be liable to occur. The volume averageparticle diameter of the inorganic fine particles can be measured by,for example, taking an electron microphotograph of the particles, andprocessing this photograph by means of an image processing analyzer[trade name “LUZEX IID”, manufactured by NIRECO Corporation].

These inorganic fine particles preferably have a hydrophobization degreeof 30 to 90% as determined by a methanol method.

No particular limitation is imposed on the amount of the inorganic fineparticles used as the external additive. However, it is within the rangeof preferably 0.1 to 5 parts by mass, more preferably 0.3 to 3 parts bymass per 100 parts by mass of the magenta-colored resin particles. Ifthe amount added is too little, the flowability of the resulting toneris lowered, and so blur may occur in some cases. If the amount is toogreat to the contrary, the flowability becomes too excessive, and so fogmay occur in some cases.

No particular limitation is imposed on the spherical or indefinableorganic fine particles. From the viewpoint of inhibiting blockingbetween particles, however, it is desirable that the glass transitiontemperature or melting point of a compound forming such organic fineparticles falls within the range of preferably 80 to 250° C., morepreferably 90 to 200° C. Examples of the compound forming the organicfine particles include methyl methacrylate polymers and styrene-methylmethacrylate copolymers.

No particular limitation is imposed on the volume average particlediameter of the organic fine particles. However, it is within the rangeof preferably 0.1 to 1 μm, more preferably 0.1 to 0.8 μm. No particularlimitation is imposed on the spheroidicity rl/rs of the organic fineparticles. However, it is within the range of preferably 1.0 to 1.3,more preferably 1.0 to 1.2. If this volume average particle diameter istoo small, it may be impossible in some cases to prevent the occurrenceof toner filming. If the volume average particle diameter is too greatto the contrary, the flowability of the resulting toner may be loweredin some cases. If the spheroidicity is too great, the transferability ofthe resulting toner may be lowered in some cases.

No particular limitation is imposed on the amount of the organic fineparticles added. However, it is within the range of preferably 0.05 to 1part by mass, more preferably 0.1 to 0.5 part by mass per 100 parts bymass of the magenta-colored resin particles. If the added amount is toolittle, it is difficult to effectively inhibit a filming phenomenon. Ifthe amount is too great to the contrary, the flowability of theresulting toner becomes poor, and so blur may possibly be liable tooccur.

3. Production Process of Magenta-Colored Resin Particles

No particular limitation is imposed on the production process of themagenta-colored resin particles (magenta toner particles) according tothe present invention, and they can be produced by, for example, (i) agrinding process, (ii) a polymerization process such as an emulsionpolymerization process or suspension polymerization process, or (iii) adissolution and suspension process.

Among these production processes, the polymerization process ispreferred in that magenta-colored resin particles, which can meet theimage quality of high resolution and the speeding-up of printing, areeasily obtained. Substantially spherical magenta-colored resin particles(i.e., magenta-colored polymer particles) obtained by the suspensionpolymerization process among the polymerization processes areparticularly preferred. The magenta-colored resin particles are madespherical, whereby the magenta-colored resin particles can be preventedfrom being broken in an image forming apparatus to lower the flowabilityof the magenta toner.

The production process of the magenta-colored resin particles by thesuspension polymerization process will hereinafter be described.Incidentally, colored resin particles obtained by the polymerizationprocess may be called polymerized toner particles or polymerized tonerin some cases.

In the suspension polymerization process, a polymerizable monomercomposition containing at least a polymerizable monomer and a magentacolorant is prepared, and the resultant polymerizable monomercomposition is polymerized in an aqueous dispersion medium to formmagenta-colored resin particles (magenta-colored polymer particles). Apolymer formed by polymerization of the polymerizable monomer becomes abinder resin. In order to form magenta-colored resin particles having acore-shell structure, a polymerizable monomer for shell is polymerizedin the presence of the magenta-colored polymer particles to form a shelllayer on each surface of the particles.

In the present invention, a magenta pigment composed of a combination ofC.I. Pigment Red 31 and C.I. Pigment Red 150 is used as the magentacolorant. In addition to the magenta colorant, additive components fortoner, such as a charge control agent and a parting agent, are containedin the polymerizable monomer composition as needed.

In the polymerization step, the polymerizable monomer composition issuspended in the aqueous dispersion medium containing a dispersionstabilizer to form droplets having a desired droplet diameter. Ingeneral, a polymerization initiator is added to the aqueous dispersionmedium before the formation of droplets of the polymerizable monomercomposition or in the course of the formation thereof to cause thepolymerization initiator to migrate into the droplets, andpolymerization is initiated by heating the suspension to a prescribedtemperature. After completing the polymerization, a remaining unreactedpolymerizable monomer is generally distilled off, and themagenta-colored resin particles are then collected by filtration,washing, dehydration and drying.

In the present invention, it is preferred that the magenta pigment bemixed with the charge control resin in advance to prepare a chargecontrol resin composition (which may be referred to as “colorant masterbatch”) from the viewpoint of enhancing the dispersibility of themagenta pigment. In this case, the magenta pigment (total amount of 2pigments) is mixed in a proportion of preferably 10 to 200 parts bymass, more preferably 20 to 150 parts by mass per 100 parts by mass ofthe charge control resin.

An organic solvent is preferably used for the preparation of the chargecontrol resin composition containing the magenta pigment and chargecontrol resin. The use of the organic solvent makes the charge controlresin soft and easy to mix with the magenta pigment. When no organicsolvent is used, it is necessary to heat both component to such atemperature that the charge control resin becomes soft to mix them. Whenan organic solvent having a low boiling point is used, the organicsolvent may be evaporated in some cases by heating. It is thuspreferable to mix both components at room temperature or while coolingthem. Since a problem of odor may be caused in some cases when theorganic solvent remains in the resulting magenta-colored resinparticles, the organic solvent is preferably volatilized off ordistilled off to remove it in the course of the preparation of thecharge control resin composition or the production of themagenta-colored resin particles.

The amount of the organic solvent used is within the range of generally0 to 100 parts by mass, preferably 5 to 80 parts by mass, morepreferably 10 to 60 parts by mass per 100 parts by mass of the chargecontrol resin. When the amount of the organic solvent used falls withinthis range, a balance between dispersibility and processing abilitybecomes excellent. The whole amount of the organic solvent may be addedeither at a time or in several portions while confirming a mixed state.

When the organic solvent is used, it is preferable that the solubilityparameter (hereinafter referred to as “SP value”) thereof be within therange of 8 to 15 [cal/cm³]^(1/2), and that the boiling point thereof bewithin the range of 50 to 150° C. If the SP value of the organic solventis smaller than 8 [cal/cm³]^(1/2), its polarity becomes too low, anddifficulty is encountered upon dissolution of the charge control resin.If the SP value is greater than 15 [cal/cm³]^(1/2) to the contrary, thepolarity becomes too high, and difficulty is encountered upondissolution of the charge control resin. On the other hand, if theboiling point of the organic solvent is lower than 50° C., the organicsolvent may be evaporated in some cases by heat generated upon themixing. If the boiling point is higher than 150° C. to the contrary,difficulty is liable to occur upon removal of the organic solvent.

Specific examples of organic solvents (SP value/boiling point) arementioned together with the SP value and boiling point and includemethanol (14.5/65° C.), ethanol (10.0/78.3° C.), propanol (11.9/97.2°C.), diethyl ketone (8.8/102° C.), di-n-propyl ketone (8.0/144° C.),di-isopropyl ketone (8.0/124° C.), methyl n-propyl ketone (8.3/102° C.),methyl isopropyl ketone (8.5/95° C.), methyl n-butyl ketone (8.5/127°C.), methyl isobutyl ketone (8.4/117° C.), toluene (8.9/110° C.),tetrahydrofuran (9.1/65° C.) methyl ethyl ketone (9.3/80° C.), acetone(9.9/56° C.) and cyclohexanone (9.9/156° C.).

These organic solvents may be used either singly or in any combinationthereof. Among these, diethyl ketone, methyl n-propyl ketone, methyln-butyl ketone, methyl ethyl ketone/methanol mixed solvent,toluene/ethanol mixed solvent and toluene/propanol mixed solvent arepreferred in view of solubility of the charge control resin and removalefficiency after the mixing.

The mixing of the magenta pigment with the charge control resin may beconducted by means of a mixer such as a roll mill, Plasticoder(manufactured by Brabender Co.), Laboplasto mill (manufactured by ToyoSeiki Co., Ltd.), kneader, single-screw extruder, twin-screw extruder,Banbury mixer or Buss Cokneader. When the organic solvent is used,problems of odor and toxicity arise, and so a mixer capable of mixing ina closed system, in which the organic solvent does not leak, ispreferably used. A mixer equipped with a torque meter is preferredbecause dispersibility can be controlled by the level of torque.

The amount of the charge control resin composition containing themagenta pigment and charge control resin to be used is within the rangeof preferably 2 to 20 parts by mass, more preferably 3 to 15 parts bymass per 100 parts by mass of the polymerizable monomer. If this amountused is too little, the charge control of the resulting magenta toner isliable to become difficult. If the amount is too great to the contrary,the resulting magenta toner undergoes moisture absorption, and so fogmay occur in some cases.

As examples of the polymerizable monomer for forming the binder resin,may be mentioned monovinyl monomers, crosslinkable monomers andmacromonomers. The polymerizable monomer is polymerized to provide abinder resin component.

Examples of the monovinyl monomers include aromatic vinyl monomers suchas styrene, vinyltoluene and α-methylstyrene; (meth)acrylic acid;derivatives of (meth)acrylic acid, such as methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, butyl(meth)acrylate,2-ethylhexyl(meth)acrylate, cyclohexyl(meth)acrylate,isobonyl(meth)acrylate, dimethylaminoethyl(meth)acrylate and(meth)acrylamide; and monoolefin monomers such as ethylene, propyleneand butylene.

The monovinyl monomers may be used either singly or in any combinationthereof. Among these monovinyl monomers, aromatic vinyl monomers, andcombinations of an aromatic vinyl monomer and a derivative of(meth)acrylic acid are preferred.

When a crosslinkable monomer and a crosslinkable polymer are usedtogether with the monovinyl monomer, hot offset of the resulting magentatoner can be effectively improved.

The crosslinkable monomer is a monomer having at least 2 vinyl groups,As specific examples thereof, may be mentioned aromatic divinylcompounds such as divinylbenzene, divinylnaphthalene and derivativesthereof; diethyl-type unsaturated carboxylic acid esters such asethylene glycol dimethacrylate and diethylene glycol dimethacrylate;compounds having 2 vinyl groups, such as N,N-divinylaniline and divinylether; and compounds having three or more vinyl groups, such aspentaerythritol triallyl ether and trimethylolpropane triacrylate.

The crosslinkable polymer is a polymer having two or more vinyl groupsin the polymer. As specific examples thereof, may be mentioned estersobtained by subjecting a polymer, such as polyethylene, polypropylene,polyester or polypropylene glycol, having two or more hydroxyl groups inits molecule, and an unsaturated carboxylic acid monomer such as acrylicacid or methacrylic acid to a condensation reaction.

These crosslinkable monomers and crosslinkable polymers may be usedeither singly or in any combination thereof. The amount of thesemonomers and polymers used is within the range of generally at most 10parts by mass, preferably 0.1 to 2 parts by mass per 100 parts by massof the monovinyl monomer.

It is preferable to use a macromonomer together with the monovinylmonomer because a balance between the storage stability andlow-temperature fixing ability of the resulting magenta toner isimproved. The macromonomer is an oligomer or polymer having apolymerizable carbon-carbon unsaturated double bond at its molecularchain terminal and a number average molecular weight generally rangingfrom 1,000 to 30,000. When the number average molecular weight fallswithin the above-described range, the fixing ability and storagestability of the resulting toner can be retained without impairing themelt properties of the macromonomer. It is hence preferable that thenumber average molecular weight be within the above range.

Examples of groups having the polymerizable carbon-carbon unsaturateddouble bond located at its molecular chain terminal include acryloyl andmethacryloyl groups. Among these, the methacryloyl group is preferredfrom the viewpoint of easiness of copolymerization.

The macromonomer is preferably that giving a polymer having a glasstransition temperature higher than that of a polymer obtained bypolymerizing the monovinyl monomer.

As examples of the macromonomer used in the present invention, may bementioned polymers obtained by polymerizing styrene, styrenederivatives, methacrylic esters, acrylic esters, acrylonitrile andmethacrylonitrile either singly or in combination of two or moremonomers thereof, and macromonomers having a polysiloxane skeleton.Among these, hydrophilic macromonomers, in particular, polymers obtainedby polymerizing methacrylic esters or acrylic esters either singly or incombination of two or more monomers thereof are preferred.

When the macromonomer is used, the amount of the macromonomer used iswithin the range of generally 0.01 to 10 parts by mass, preferably 0.03to 5 parts by mass, more preferably 0.05 to 1 part by weight per 100parts by weight of the monovinyl monomer. The macromonomer is preferablyused in the amount within the above-described range because the storagestability of the resulting magenta toner can be retained withoutdeteriorating the fixing ability thereof.

The suspension polymerization is generally conducted in an aqueousdispersion medium containing a dispersion stabilizer. As examples of thedispersion stabilizers, may be mentioned metallic compounds, such assulfates such as barium sulfate and calcium sulfate; carbonates such asbarium carbonate, calcium carbonate and magnesium carbonate; phosphatessuch as calcium phosphate; and metal oxides such as aluminum oxide andtitanium oxide; and besides, metal hydroxides such as aluminumhydroxide, magnesium hydroxide and ferric hydroxide; water-solublepolymers such as polyvinyl alcohol, methyl cellulose and gelatin; andsurfactants such as anionic surfactants, nonionic surfactants andamphoteric surfactants. These dispersion stabilizers may be used eithersingly or in any combination thereof.

Among these dispersion stabilizers, colloid of a metallic compound,particularly, a hardly water-soluble metal hydroxide is preferredbecause the particle diameter distribution of the colored polymerparticles to be formed can be narrowed, and remaining tendency of thedispersion stabilizer after washing is little, and an image can bebrightly or sharply reproduced.

The colloid of the hardly water-soluble metal hydroxide preferably has aparticle diameter distribution that a particle diameter D₅₀ that thecumulative particle number counting from a smaller particle diameterside is 50% is at most 0.5 μm, and a particle diameter D₉₀ that thecumulative particle number is 90% is at most 1 μm. If the particlediameter of the colloid is too great, the stability of thepolymerization is broken, and the storage stability of the resultingtoner is deteriorated.

The dispersion stabilizer is used in a proportion of generally 0.1 to 20parts by mass per 100 parts by mass of the polymerizable monomer. Thedispersion stabilizer is preferably used in the proportion fallingwithin the above-described range because sufficient polymerizationstability is achieved, formation of polymer aggregates is inhibited, andcolored polymer particles having a desired particle diameter can beobtained.

As examples of the polymerization initiator, may be mentionedpersulfates such as potassium persulfate and ammonium persulfate; azocompounds such as 4,4′-azobis-(4-cyanovaleric acid),2,2′-azobis(2-methyl-N-(2-hydroxyethyl)propionamide,2,2′-azobis(2-amidinopropane) dihydrochloride,2,2′-azobis(2,4-dimethylvaleronitrile) and 2,2′-azobisisobutyronitrile;and peroxides such as di-t-butyl peroxide, dicumyl peroxide, lauroylperoxide, benzoyl peroxide, t-butyl peroxy-2-ethylhexanoate, t-hexylperoxy-2-ethylhexanoate, t-butyl peroxypivalate, di-isopropylperoxydicarbonate, di-t-butyl peroxyisophthalate,1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate and t-butylperoxyisobutyrate. Redox initiators composed of combinations of thesepolymerization initiators with a reducing agent may also be used.

It is preferable to select an oil-soluble polymerization initiatorsoluble in the used polymerizable monomer from among these. Awater-soluble polymerization initiator may also be used in combinationwith the above-described initiator as needed. The polymerizationinitiator is used in a proportion of generally 0.1 to 20 parts by mass,preferably 0.3 to 15 parts by mass, more preferably 0.5 to 10 parts bymass per 100 parts by weight of the polymerizable monomer.

The polymerization initiator may be added into the polymerizable monomercomposition in advance. In order to prevent premature polymerization in,for example, the step of preparing the polymerizable monomer compositionor the step of forming the droplets, however, the polymerizationinitiator may be directly added into a suspension in the course of theformation of droplets of the polymerizable monomer composition or aftercompletion of the formation of the droplets in the case of suspensionpolymerization, or an emulsion after completion of a emulsifying step inthe case of emulsion polymerization.

Upon the polymerization, a molecular weight modifier is preferably used.As examples of the molecular weight modifier, may be mentionedmercaptans such as t-dodecylmercaptan, n-dodecylmercaptan,n-octylmercaptan and 2,2,4,6,6-pentamethylheptane-4-thiol; andhalogenated hydrocarbons such as carbon tetrachloride and carbontetrabromide. These molecular weight modifiers may be added before theinitiation of the polymerization or in the course of the polymerization.The molecular weight modifier is used in a proportion of generally 0.01to 10 parts by mass, preferably 0.1 to 5 parts by mass per 100 parts byweight of the polymerizable monomer.

Examples of a process for producing the magenta-colored resin particleshaving the core-shell structure include processes such as a spray dryingprocess, interfacial reaction process, in-situ polymerization processand phase separation process. More specifically, magenta-colored resinparticles obtained by a grinding process, polymerization process,association process, phase-inversion emulsification process or the likeare used as core particles, a shell layer is formed on the surface ofeach of the core particles, whereby the magenta-colored resin particleshaving the core-shell structure can be obtained. Among these productionprocesses, in-situ polymerization process and phase-inversionemulsification process are preferred from the viewpoint of productionefficiency.

The production process of the magenta-colored resin particles having thecore-shell structure by the in-situ polymerization process willhereinafter be described.

A polymerizable monomer (polymerizable monomer for shell) for formingshell layers and a polymerization initiator are added into an aqueousdispersion medium with core particles dispersed therein and polymerizedto form a shell layer composed of a polymer on the surface of each ofthe core particles, whereby magenta-colored resin particles of thecore-shell structure can be obtained.

As specific processes for forming the shell layers, may be mentioned aprocess, in which a polymerizable monomer for shell is added into areaction system of a polymerization reaction conducted for obtainingcore particles, and the polymerization reaction is continued; astep-wise process, in which core particles obtained in a separatereaction system are charged into an aqueous dispersion medium, and apolymerizable monomer for shell is added to the resultant dispersion toconduct polymerization, and the like.

The polymerizable monomer for shell may be added into the reactionsystem at a time, or may be added continuously or intermittently bymeans of a pump such as a plunger pump.

As the polymerizable monomer for shell, monomers respectively formingpolymers having a glass transition temperature exceeding 80° C., such asstyrene, acrylonitrile and methyl methacrylate, are preferably usedeither singly or in combination of two or more monomers thereof.

A water-soluble polymerization initiator is preferably added upon theaddition of the polymerizable monomer for shell because themagenta-colored resin particles of the core-shell structure are easy tobe obtained. It is considered that when the water-soluble polymerizationinitiator is added upon the addition of the polymerizable monomer forshell, the water-soluble polymerization initiator enters in the vicinityof each surface of the core particles to which the polymerizable monomerfor shell has migrated, and consequently a polymer (shell) layer is easyto be formed on each surface of the core particles.

As examples of the water-soluble polymerization initiator, may bementioned persulfates such as potassium persulfate and ammoniumpersulfate; and azo type initiators such as2,2′-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide] and2,2′-azobis-[2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide. Theamount of the water-soluble polymerization initiator used is within therange of generally 0.1 to 50 parts by mass, preferably 1 to 30 parts bymass per 100 parts by mass of the polymerizable monomer for shell.

4. Production Process of Magenta Toner

A magenta toner can be produced as a non-magnetic one-componentdeveloper by adding an external additive to magenta-colored resinparticles (magenta toner particles). A magenta toner can be produced asa two-component developer by mixing magenta-colored resin particles withcarrier particles. A process, in which magenta-colored resin particles(including magenta-colored resin particles of the core-shell structure)are synthesized by the suspension polymerization process, and anexternal additive is added to the magenta-colored resin particles toproduce a non-magnetic one-component developer (magenta toner), ispreferred.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following examples and comparative examples. However, the presentinvention is not limited to these examples alone. All designations of“part” or “parts” and “%” mean part or parts by mass and % by massunless expressly noted.

Various physical properties and characteristics in the present inventionwere evaluated in accordance with the following respective methods.

1. Physical Properties of Toner

(1) Average Particle Diameter and Particle Diameter Distribution ofToner Particles:

The volume average particle diameter dv and particle diameterdistribution, i.e., a ratio dv/dp of the volume average particlediameter to the number average particle diameter dp, of toner particles(colored polymer particles) were measured by means of a Multisizer(manufactured by Beckmann Coulter Co.). The measurement by thisMultisizer was conducted under the following conditions:

aperture diameter: 100 μm;

medium: Isothone II, concentration: 10%; and

the number of particles measured: 100,000 particles.

(2) Spheroidicity:

A photograph of toner particles was taken by a scanning electronmicroscope, and the photograph was read in an image processor, intowhich a Nexus 9000 type software had been incorporated, to determine avalue rl/rs by dividing a length rl of each toner particle by a breadthrs thereof. The number of particles measured was 100 particles, and anaverage value of the measured values was regarded as an averagespheroidicity.

2. Evaluation of Toner Properties

(1) Charge Level:

A charge level of a developer (toner) comprising toner particles wasmeasured under respective environments of L/L environment of 10° C. intemperature and 20% in humidity, N/N environment of 23° C. intemperature and 50% in humidity, and H/H environment of 35° C. intemperature and 80% in humidity.

More specifically, a printer on the market (manufactured by Oki DataCorporation, trade name “MICROLINE 3010C”) of the non-magneticone-component development system was charged with a toner sample andleft to stand for a day under the respective environments describedabove, and a half-tone print pattern was then printed by 5 sheets.Thereafter, the toner on a development roller was sucked by a suctiontype charge level meter to measure a charge level per unit mass from acharge level of the toner sucked and an amount of the toner sucked.

(2) Storage Stability:

About 20 g of a toner sample were precisely weighed and placed in aclosed bottle. The weight of the toner at this time was regarded as W1.The bottle was sunk into a constant-temperature water bath controlled to55° C. The bottle was taken out of the water bath after 8 hours hadelapsed, and the toner contained in the bottle was transferred to a42-mesh sieve so as not to destroy the structure of the toner as much aspossible. The sieve was vibrated for 30 seconds by means of apowder-measuring device (“Powder Tester”, trade name; manufactured byHosokawa Micron Corporation) with vibration intensity preset to 4.5. Theweight of the toner remaining on the sieve was then measured to regardit as the weight of the toner aggregated. A percentage (% by weight) ofthe weight W2 of the aggregated toner to the weight W1 of the tonersample was calculated out in accordance with an equation [(W2/W1)×100].The value was used as an index to the storage stability of the tonersample. The smaller the value, the higher the storage stability.

(3) Evaluation of Image Quality:

i) Color Tone:

After paper for printing was set in the above-described printer, a tonersample was charged into a developing device, and the printer was left tostand for a day under (N/N) environment of 23° C. in temperature and 50%in humidity, solid printing was conducted. With respect to the paper, onwhich the solid printing had been conducted, the L*a*b* color spacethereof was measured by means of a spectroscopic color-difference meter(manufactured by Nippon Denshoku K. K., model name “SE2000”). Withrespect to a hue difference from the magenta of Japan Color, a colortone of Japan Color Standard Paper measured likewise and a color toneobtained by printing with the toner sample are represented ascoordinates of the L*C*H* color space to calculate out the huedifference ΔH in accordance with the following equation:ΔH*=[(ΔE*)²−(ΔL*)²−(ΔC*)²]^(1/2)wherein

-   -   ΔE*: a color difference by the L*a*b* color space,    -   ΔL*: a lightness index difference between 2 object colors in the        L*a*b* color space, and    -   ΔC*: a difference between 2 object colors, ab chromas in the        L*a*b* color space.

The amount of the toner attached to the surface of the paper in thesolid printing was controlled so as to be about 0.45 mg/cm².

ii) Printing Density:

After paper for printing was set in the above-described printer, a tonersample was charged into a developing device, and the printer was left tostand for a day under H/H environment of 35° C. in temperature and 80%in humidity, printing was continuously conducted from the beginning at adensity of 5%, and solid printing was conducted upon printing on the20,000-th paper sheet. With respect to the solid-printed paper sheet,the printing density was measured by means of a color reflectiondensitometer (manufactured by X-Light Co., model name “404A”).

iii) Fog:

After paper for printing was set in the above-described printer, a tonersample was charged into a developing device, and the printer was left tostand for a day under each environment of L/L environment of 10° C. intemperature and 20% in humidity, N/N environment of 23° C. intemperature and 50% in humidity and H/H environment of 35° C. intemperature and 80% in humidity, printing was continuously conducted ata density of 5%. After conducting printing on 20,000 sheets of paper,blank printing of a sheet of paper was conducted, the blank printing wasstopped on the way, and a toner in an unprinted area on a photosensitivemember after development was taken out using a pressure-sensitiveadhesive tape (product of Sumitomo 3M Limited, trade name “ScotchMending Tape 810-3-18”). This adhesive tape was applied to new paper forprinting. The color tone B of the paper for printing, to which thisadhesive tape had been applied, was measured by the spectroscopiccolor-difference meter, and the color tone A of paper for printing, towhich only a pressure-sensitive adhesive tape had been applied, wasmeasures likewise. Their color tones were represented as coordinates ofthe L*a*b* color space to calculate out a color difference ΔE to regardit as a fog value. The smaller the value, the less the fog.

(4) Fixing Temperature:

The above-described printer was modified in such a manner that thetemperature of a fixing roll can be varied. This modified printer wasused to vary the temperature of the fixing roll at intervals of 5° C.,thereby determining a fixing rate of a toner sample at each temperature.

In order to stabilize the temperature of the fixing roll varied, themodified printer was left to stand for at least 5 minutes at eachtemperature set, and solid printing was then conducted on paper forprinting by the modified printer. With respect to the solid-printed areaof the paper printed, the fixing rate was calculated from a ratio ofprinting densities before and after a peeling operation using a tape.More specifically, assuming that the image density before the peeling ofthe tape is ID_(before), and the image density after the peeling of thetape is ID_(after), the fixing rate is calculated out in accordance withthe following equation:Fixing rate (%)=(ID _(after) /ID _(before))×100

The peeling operation of the tape is a series of operation that apressure-sensitive adhesive tape (product of Sumitomo 3M Limited, tradename “Scotch Mending Tape 810-3-18”) is applied to a measuring area ofpaper for test to cause the tape to adhere to the paper by pressing thetape under a fixed pressure, and the adhesive tape is then peeled at aconstant rate in a direction along the paper.

In this fixing test, a temperature of the fixing roll, at which a fixingrate of the toner amounted to 80%, was defined as a fixing temperatureof the toner.

(5) Hot Offset Temperature:

The temperature of the fixing roll was varied in the same manner as inthe measurement of the fixing temperature to conduct solid printing,thereby measuring a temperature, at which hot offset occurred. Thetemperature of the fixing roll, at which hot offset occurred, wasdefined as a hot offset temperature of the toner.

Example 1

1. Preparation of Charge Control Resin Composition (Pigment MasterBatch):

In 100 parts of a charge control resin (weight average molecular weight:18,000; glass transition temperature: 67° C.) obtained by polymerizing amonomer mixture composed of 82% of styrene, 11% of n-butyl acrylate and7% of 2-acrylamido-2-methylpropanesulfonic acid, were dispersed 24 partsof methyl ethyl ketone and 6 parts of methanol, and the dispersion waskneaded by rolls while cooling it. At the time the charge control resinwound around the rolls, 55 parts of C.I. Pigment Red 31 and 45 parts ofC.I. Pigment Red 150 were gradually added as a magenta pigment, and theresultant mixture was kneaded for 1 hour to prepare a charge controlresin composition containing the magenta pigment. At this time, a nipbetween the rolls was preset to 1 mm at the beginning of the kneading,gradually widened after this and finally widened to 3 mm. An organicsolvent (4/1 mixed solvent of methyl ethyl ketone/methanol) wasadditionally added several times according to the kneaded state of thecharge control resin.

A part of the charge control resin composition was taken out, andtoluene was added to this part to dissolve it, thereby preparing a 5%toluene solution of the charge control resin composition. A doctor bladeat an interval of 30 μm was used to apply the 5% toluene solution on toa glass plate and dried to form a sheet of the charge control resincomposition. This sheet was observed through a optical microscope of 400magnifications. As a result, no magenta pigment particle having a lengthof 0.2 μm or greater was present in a visual field 100 μm square.

2. Preparation of Aqueous Dispersion Medium Containing DispersionStabilizer:

An aqueous solution with 6.9 parts of sodium hydroxide (alkali metalhydroxide) dissolved in 50 parts of ion-exchanged water was graduallyadded to an aqueous solution with 9.8 parts of magnesium chloride(water-soluble polyvalent metal salt) dissolved in 250 parts ofion-exchanged water under stirring to prepare a dispersion of magnesiumhydroxide colloid (colloid of hardly water-soluble metal hydroxide). Theparticle diameter distribution of the colloid prepared was measured. Asa result, the particle diameter was 0.38 μm in terms of D₅₀ (50%cumulative value of number particle diameter distribution) and 0.82 μmin terms of D₉₀ (90% cumulative value of number particle diameterdistribution).

3. Preparation of Polymerizable Monomer Composition for Core:

A polymerizable monomer for core composed of 80.5 parts of styrene, 19.5parts of n-butyl acrylate, 0.5 parts of divinylbenzene and 0.25 parts ofa polymethacrylic ester macromonomer (product of Toagosei ChemicalIndustry Co., Ltd., trade name “AA6”) were mixed under stirring with 12parts of the above-described charge control resin composition, 2 partsof t-dodecylmercaptan and 10 parts of dipentaerythritol hexamyristateinto a uniform dispersion, thereby obtaining a polymerizable monomercomposition for core.

4. Preparation of Polymerizable Monomer for Shell:

One part of methyl methacrylate and 100 parts of water were subjected toa finely dispersing treatment by an ultrasonic emulsifier to obtain anaqueous dispersion of a polymerizable monomer for shell. The diameter ofdroplets of the polymerizable monomer for shell was measured by means of(SALD2000A Model, manufactured by Shimadzu Corporation) and found to be1.6 μm in terms of D₉₀.

5. Step of Forming Droplets:

After the polymerizable monomer composition for core obtained in thepreparation step 3 was poured into the colloidal dispersion of magnesiumhydroxide obtained in the preparation step 2, and the mixture wasstirred until droplets became stable, 6 parts of t-butylperoxy-2-ethylhexanoate (product of Nippon Oil & Fats Co., Ltd., tradename “Perbutyl O”) were added. Thereafter, the resultant dispersion wasstirred 30 minutes at 15,000 rpm under high shearing force by means of adispersing machine (manufactured by Ebara Corporation, trade name “EbaraMilder”) to form finer droplets of the polymerizable monomercomposition.

6. Polymerization Step:

A reactor equipped with an agitating blade was charged with the aqueousdispersion of the polymerizable monomer composition for core obtained inthe step 5 to initiate a polymerization reaction at 90° C. At the time aconversion into a polymer reached almost 100%, the reaction mixture wassampled to measure a volume average particle diameter of colored polymerparticles (core particles). As a result, it was 7.4 μm.

The aqueous dispersion of the polymerizable monomer for shell obtainedin the step 4 and 0.2 parts of2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] (product of WakoPure Chemical Industries, Ltd., trade name “VA-086”) dissolved in 65parts of distilled water were charged into the reactor, and thepolymerization was then continued for 8 hours. After the temperature ofthe aqueous dispersion was lowered to 80° C. while retaining agitation,and nitrogen gas was introduced while retaining this temperature todistil off unreacted polymerizable monomers, the residue was cooled toobtain an aqueous dispersion of magenta toner particles at pH 9.5.

7. Washing and Collecting Step:

The pH of the system was adjusted to 5 or lower with sulfuric acid whilestirring the aqueous dispersion of the magenta toner particles obtainedin the step 6 to conduct acid washing (25° C., 10 minutes). After waterwas then separated by filtration, 500 parts of ion-exchanged water werenewly added to form a slurry again, and the slurry was washed withwater. Thereafter, the dehydration and water washing were repeatedseveral times, and solids were then separated by filtration. The solidswere dried at 45° C. for 2 days by a dryer to obtain dry magenta tonerparticles.

The volume average particle diameter dv of the dry magenta tonerparticles was 7.4 μm, a particle diameter distribution dv/dp was 1.23,and an average spheroidicity rl/rs was 1.1. The magenta toner particlesare colored polymer particles having a core-shell structure.

7. Preparation of Non-Magnetic One-Component Developer:

Into 100 parts of the magenta toner particles obtained above were mixed0.3 parts of cubical calcium carbonate (product of Maruo Calcium Co.,Ltd., trade name “CUBE-03BHS”) having a volume average particle diameterof 0.3 μm, 0.5 parts of indefinable finely particulate silica (productof Nippon Aerosil Co., Ltd., trade name “RX-300”) subjected to ahydrophobicity-imparting treatment and 2.0 parts of indefinable finelyparticulate silica (product of Nippon Aerosil Co., Ltd., trade name“RX-50”) subjected to a hydrophobicity-imparting treatment, by means ofa Henschel mixer to prepare a magenta toner (non-magnetic one-componentdeveloper).

The properties of the toner thus obtained and the results of theevaluation as to image quality are shown in Table 1.

Comparative Example 1

1. Preparation of Aqueous Dispersion Medium Containing DispersionStabilizer:

A 2-liter 4-necked flask equipped with a high-speed agitating machine,TK type homomixer (manufactured by Tokushu Kika Kogyo Co., Ltd.) wascharged with 360 parts of ion-exchanged water and 430 parts of a 0.1mol/liter aqueous solution of Na₃PO₄. The rotating speed of thehigh-speed agitating machine was preset to 12,000 rpm, and the systemwas heated to 65° C. while stirring the aqueous solution. To thisaqueous solution, were added 34 parts of a 1.0 mol/liter aqueoussolution of CaCl₂ to prepare an aqueous dispersion with a hardlywater-soluble dispersion stabilizer Ca₃(PO₄)₂ finely dispersed therein.

2. Preparation of Pigment-Dispersed Composition:

A mixture composed of 43 parts of styrene, 4.5 parts of C.I. Pigment Red122 (content of calcium abietate: 0.5 parts) treated with calciumabietate, 2.3 parts of C.I. Pigment Red 150 (content of calciumabietate: 0.3 parts) treated with calcium abietate, 3 parts of a chargecontrol agent (product of Orient Chemical Industries Ltd., trade name“E-89”) and 6 parts of a polyester resin (peak molecular weight=5,000,acid value=20 mg KOH/g) was dispersed for 3 hours by means of anAttritor (manufactured by Mitsui Kinzoku Kogyo Co., Ltd.) to prepare apigment-dispersed composition.

3. Preparation of Polymerizable Monomer Composition:

A container was charged with 40 parts of styrene, 17 parts of n-butylacrylate, 0.2 parts of divinylbenzene, 15 parts of a wax component(higher alcohol wax, melting point=70° C.) and the whole amount (58.8parts) of the pigment-dispersed composition obtained in the preparationstep 2, the contents were dispersed and dissolved while being heated to70° C., and 3 parts of 2,2′-azobis(2,4-dimethyl-valeronitrile) wereadded to prepare a polymerizable monomer composition.

4. Step of Forming Droplets:

The polymerizable monomer composition obtained in the preparation step 3was poured into the aqueous dispersion medium obtained in thepreparation step 1, and the resultant mixture was stirred for 5 minuteswhile retaining the rotating speed of a high-speed agitating machine at15,000 rpm under a nitrogen gas atmosphere at an internal temperature of65° C. to form droplets of the polymerizable monomer composition.

5. Polymerization Step:

After the step of forming the droplets, the agitating machine waschanged to that equipped with a paddle-agitating blade to conductpolymerization with the temperature kept at the same temperature whilestirring at 200 rpm, and the polymerization reaction was completed atthe time a conversion of the polymerizable monomer into a polymerreached almost 100%.

6. Washing and Collecting Step:

After completion of the polymerization, unreacted polymerizable monomerswere distilled off heating under reduced pressure. After cooling, dilutehydrochloric acid was added to the reaction mixture to dissolve thehardly water-soluble dispersing agent. After washing with water wasconducted several times repeatedly, a drying treatment was conducted toobtain magenta toner particles.

7. Preparation of Non-Magnetic One-Component Developer:

Into 100 parts of the magenta toner particles obtained above were mixed1.5 parts of indefinable finely particulate silica (product of NipponAerosil Co., Ltd., trade name “R-202”) subjected to ahydrophobicity-imparting treatment, by means of a Henschel mixer toprepare a magenta toner (non-magnetic one-component developer).

The properties of the magenta toner thus obtained and the results of theevaluation as to image quality are shown in Table 1.

Comparative Example 2

1. Preparation of Aqueous Dispersion Medium Containing DispersionStabilizer:

A 2-liter 4-necked flask equipped with a high-speed agitating machine,TK type homomixer (manufactured by Tokushu Kika Kogyo Co., Ltd.) wascharged with 360 parts of ion-exchanged water and 430 parts of a 0.1mol/liter aqueous solution of Na₃PO₄. The rotating speed of thehigh-speed agitating machine was preset to 12,000 rpm, and the systemwas heated to 65° C. To this aqueous solution, were added 34 parts of a1.0 mol/liter aqueous solution of CaCl₂ to prepare an aqueous dispersionwith a hardly water-soluble dispersion stabilizer Ca₃(PO₄)₂ finelydispersed therein.

2. Preparation of Polymerizable Monomer Composition:

After a mixture composed of 83 parts of styrene, 17 parts of n-butylacrylate, 6.6 parts of C.I. Pigment Red 150 (content of calciumabietate: 0.6 parts) treated with calcium abietate, 5 parts of apolyester resin (Mw=25,000), 0.03 parts of d-t-butylsalicylic acid and15 parts of an ester wax (Mn=1,000, Mw/Mn=1.9) was dispersed for 3 hoursby means of a dispersing machine (manufactured by Mitsui Kinzoku KogyoCo., Ltd., trade name “Attritor”), 3 parts of2,2′-azobis(2,4-dimethylvaleronitrile) were added to prepare apolymerizable monomer composition.

3. Step of Forming Droplets:

The polymerizable monomer composition obtained in the preparation step 2was poured into the aqueous dispersion medium obtained in thepreparation step 1, and the resultant mixture was stirred for 5 minuteswhile retaining the rotating speed of a high-speed agitating machine at15,000 rpm under a nitrogen gas atmosphere at an internal temperature of60° C. to form droplets of the polymerizable monomer composition.

4. Polymerization Step:

Thereafter, the agitating machine was changed to that equipped with apaddle-agitating blade to conduct polymerization for 5 hours with thetemperature kept at the same temperature while stirring at 200 rpm.

5. Washing and Collecting Step:

After completion of the polymerization, sodium hydrogencarbonate wasadded into the aqueous dispersion medium to adjust the pH to 11 again, 1part of potassium persulfate that is a water-soluble initiator wasadditionally added to conduct distillation for 5 hours under a reducedpressure of 350 mmHg at an internal temperature of 80° C. After cooling,dilute hydrochloric acid was added to adjust the pH of the aqueousdispersion medium to 1.2, thereby dissolving the hardly water-solubledispersing agent. After filtration and washing with water were conductedseveral times repeatedly, and solid-liquid separation was performed, theresultant solids were subjected to a drying treatment to obtain magentatoner particles.

6. Preparation of Non-Magnetic One-Component Developer:

Into 100 parts of the magenta toner particles obtained above were added0.7 parts of titanium oxide subjected to a hydrophobicity-impartingtreatment and having an average particle diameter of 30 nm and 0.7 partsof finely particulate silica (product of Nippon Aerosil Co., Ltd., tradename “R-202”) subjected to a hydrophobicity-imparting treatment, andthese components were mixed by means of a Henschel mixer to prepare amagenta toner (non-magnetic one-component developer).

The properties of the magenta toner thus obtained and the results of theevaluation as to image quality are shown in Table 1.

Comparative Example 3

1. Preparation of Ground Toner:

After 100 parts of a polyester resin (condensation polymer ofpropoxylated bisphenol A and fumaric acid, acid value: 10.8 mg KOH/g), 4parts of a charge control agent (aluminum compound ofdi-t-butylsalicylic acid) having negatively charging ability and 5 partsof C.I. Pigment Red 31 were sufficiently premixed by means of a Henschelmixer, the resultant mixture was melt-kneaded by a twin-screw extruder.After cooling the kneaded product, it was coarsely ground to the size ofabout 1 to 2 mm by means of a hammer mill, and the resultant coarseparticles were finely ground by an air-jet type mill. Finely groundpowder and coarsely ground powder were simultaneously precisely removedfrom the finely ground product by a multi-division classifier, therebyobtaining magenta toner particles having an average particle diameter of8.0 μm.

2. Preparation of Non-Magnetic One-Component Developer:

Into 100 parts of the magenta toner particles obtained above were mixed1.5 parts of finely particulate silica (product of Nippon Aerosil Co.,Ltd., trade name “R-202”) subjected to a hydrophobicity-impartingtreatment by means of a Henschel mixer to prepare a magenta toner(non-magnetic one-component developer).

The properties of the magenta toner thus obtained and the results of theevaluation as to image quality are shown in Table 1.

TABLE 1 Japan Color Ex. Comp. Ex. Magenta 1 1 2 3 Magenta pigment PR31PR122 PR150 PR31 PR150 PR150 Physical properties of toner particlesVolume average 7.2 8.2 7.8 8.0 diameter (μm) particle Particle diameter1.23 1.39 1.42 1.47 distribution Average spheroidicity 1.13 1.20 1.221.53 Properties of toner Charge level (μC/g) L/L environment −42 −60 −30−35 N/N environment −32 −55 −25 −25 H/H environment −35 −18 −14 −20Storage stability (%) 2.0 3.5 4.5 50 Evaluation of image quality Hue L*44.26 43.92 51.17 49.20 48.72 a* 69.06 66.49 64.56 59.83 57.43 b* −9.37−5.65 −16.49 4.21 −1.73 Hue difference (ΔH) 3.24 7.85 13.23 6.62 Amountattached on to 0.45 0.45 0.46 0.44 paper (mg/cm²) Printing density 1.491.34 1.39 1.31 Fog L/L environment 0.3 1.5 0.7 0.4 N/N environment 0.11.1 1.5 1.2 H/H environment 0.7 2.4 6.2 1.1 Fixing temperature 140 170170 140 (° C.) Hot offset temperature 200 200 200 150 (° C.) (Note)PR31: C.I. Pigment Red 31 PR122: C.I. Pigment Red 122 PR150: C.I.Pigment Red 150

From the results shown in Table 1, the following facts are known.

It is known that the magenta toner of Comparative Example 1 making useof the mixture of C.I. Pigment Red 122 and C.I. Pigment Red 150 as amagenta pigment is much removed in hue from the magenta of the JapanColor standard paper printed by inks, low in printing density, liable tocause fog under the respective environments and also poor inlow-temperature fixing ability.

It is known that the magenta toner of Comparative Example 2 making useof C.I. Pigment Red 150 alone as a magenta pigment is much removed inhue from the magenta of the Japan Color standard paper printed by inks,low in printing density, liable to cause fog under both N/N environmentand H/H environment and also poor in low-temperature fixing ability.

It is known that the magenta toner of Comparative Example 3 making useof C.I. Pigment Red 31 alone as a magenta pigment is much removed in huefrom the magenta of the Japan Color standard paper printed by inks, lowin printing density, liable to cause fog under both N/N environment andH/H environment, easy to cause hot offset and also poor in storagestability of the toner.

On the other hand, the magenta toner of Example 1 according to thepresent invention can form an image of a hue near to the magenta of theJapan Color standard paper printed by inks by using, as the magentapigment, C.I. Pigment Red 31 and C.I. Pigment Red 150 in combination,and is high in printing density, hard to cause fog, excellent inlow-temperature fixing ability and hard to cause hot offset even afterconducting printing of 20,000 sheets as a durability test or even underany environment.

INDUSTRIAL APPLICABILITY

According to the present invention, there can be provided magenta tonerswhich are high in printing density, causes no fog and can reproduce amagenta hue equivalent to printing with inks, and a production processthereof. The magenta toners according to the present invention can fixat a low temperature, and are hard to cause fog even under severeenvironments of low temperature and low humidity, and high temperatureand high humidity. Further, the magenta toners according to the presentinvention prevent their magenta toner particles from breaking in animage forming apparatus to low flowability, prevents images obtained byprinting from fading and have little possibility of causing anenvironmental problem even when transfer media, on which images havebeen formed with the toners, are incinerated.

1. A magenta toner having magenta-colored resin particles comprising atleast a binder resin and a magenta colorant, wherein the magentacolorant is a magenta pigment composed of the combination of C.I.Pigment Red 31 and C.I. Pigment Red 150 within the range from 40:60 to70:30 in terms of a mass ratio, and the magenta-colored resin particleshave a volume average particle diameter dv within the range from 3.0 to12.0 μm.
 2. The magenta toner according to claim 1, wherein themagenta-colored resin particles contains the magenta pigment composed ofthe combination of C.I. Pigment Red 31 and C.I. Pigment Red 150 in theproportion of 1 to 10 parts by mass per 100 parts by mass of the binderresin.
 3. The magenta toner according to claim 1, wherein themagenta-colored resin particles further comprises a charge control resinas a charge control agent.
 4. The magenta toner according to claim 3,wherein the charge control resin has a weight average molecular weightwithin the range from 2,000 to 50,000 and a glass transition temperaturewithin the range from 40 to 80° C.
 5. The magenta toner according toclaim 1, wherein the magenta-colored resin particles further comprises aparting agent.
 6. The magenta toner according to claim 1, wherein themagenta-colored resin particles are magenta-colored resin particleshaving a core-shell structure.
 7. The magenta toner according to claim1, wherein the magenta-colored resin particles have a particle diameterdistribution dv/dp within the range from 1.0 to 1.3 in terms of theratio of the volume average particle diameter dv to the number averageparticle diameter dp.
 8. The magenta toner according to claim 1, whereinthe spheroidicity rl/rs represented by the ratio of the length rl ofeach magenta-colored resin particle to the breadth rs thereof is withinthe range from 1.0 to 1.3.
 9. The magenta toner according to claim 1,which comprises the magenta-colored resin particles and an externaladditive.
 10. The magenta toner according to claim 9, wherein theexternal additive is composed of hexahedral inorganic fine particleshaving the volume average particle diameter of 0.05 to 10 μm.
 11. Themagenta toner according to claim 9, wherein the external additive iscomposed of spherical or indefinable inorganic fine particles having thevolume average particle diameter from 5 to 500 nm.
 12. The magenta toneraccording to claim 9, wherein the external additive is composed oforganic fine particles having the volume average particle diameter from0.1 to 1 μm.
 13. A process for producing a magenta toner havingmagenta-colored resin particles, which comprises: Step 1 of preparing apolymerizable monomer composition containing at least a polymerizablemonomer and a magenta colorant; and Step 2 of polymerizing thepolymerizable monomer composition in an aqueous dispersion medium toform the magenta-colored resin particles; wherein a magenta pigmentcomposed of a combination of C.I. Pigment Red 31 and C.I. Pigment Red150 within the range from 40:60 to 70:30 in terms of a mass ratio isused as the magenta colorant, and whereby obtaining magenta-coloredresin particles having a volume average particle diameter dv within therange from 3.0 to 12.0 μm.
 14. The production process according to claim13, wherein, in the Step 1, the polymerizable monomer compositionfurther contains a charge control agent.
 15. The production processaccording to claim 14, wherein, in the Step 1: a charge control resin isused as the charge control agent; a charge control resin compositioncontaining the magenta pigment and the charge control resin is preparedin advance; and the charge control resin composition is contained in thepolymerizable monomer composition.
 16. The production process accordingto claim 15, wherein the magenta pigment and the charge control resinare mixed in the presence of an organic solvent to prepare the chargecontrol resin composition.
 17. The production process according to claim13, wherein, in the Step 1, the polymerizable monomer further comprisesa parting agent.
 18. The production process according to claim 13,wherein, in the Step 2, a polymerizable monomer for shell is furtherpolymerized in the presence of the magenta-colored resin particlesformed, to form magenta-colored resin particles with a core-shellstructure.
 19. The production process according to claim 13, whichfurther comprises the step of adding an external additive to themagenta-colored resin particles obtained in Step 2.